Washington (CNN) -- The nation's capital city took a major step Friday toward legalizing same-sex marriage.

District of Columbia Mayor Adrian Fenty signed a measure recognizing such marriages as legal. The district council overwhelming passed the bill Tuesday, following a similar vote December 1.

Fenty signed the measure at All Souls Church, a Unitarian Universalist house of worship in the northwest part of the district that is known for its diversity and for the welcoming of same-sex couples.

The measure now goes to Congress for a 30-day review period, but it's considered unlikely that the Democratic majority on Capitol Hill would block the bill. By law, Congress has the right to review and overturn laws created by the District of Columbia's council.

If the measure becomes law, the district will join Massachusetts, Connecticut, Vermont and Iowa in legalizing same-sex marriages. A law legalizing such marriages in New Hampshire takes effect January 1.

Earlier this year, lawmakers in Maine approved a measure legalizing same-sex marriages, but voters in the state last month passed a referendum to overturn the new law. Last week, New York's state Senate defeated a bill that would legalize such marriages. A similar bill stalled last week in New Jersey's state Senate.

The measure "reinforces the legal equality and religious freedoms to which all D.C. residents are entitled," the organization's president, Joe Solmonese, said in a written statement.

The National Organization for Marriage, which opposes same-sex marriage, promised earlier in the week that "the fight is not over."

"Politicians on the city council are acting as if they have the right through legislation to deprive citizens of D.C. of their core civil right to vote, but we will not let them get away with it," said Brian Brown, the organization's executive director.

"We will go to Congress, we will go to the courts, we will fight for the people's right to vote," he said.

Opposition to the legislation also came from the Catholic Church's Archdiocese of Washington, which has said that the measure could restrict the church's ability to provide charity services, apparently because the church might cut back on services rather than comply with the measure's requirements.

CNN Deputy Political Director Paul Steinhauser contributed to this report.